A major challenge facing biomedical research is to determine how the network of interactions involving gene regulatory proteins is controlled in the context of the natural environment inside living cells, and to understand how disease processes affect these activities. The expression of the prolactin (PRL) gene in anterior pituitary lactotrope cells is a proven model system to define the molecular mechanisms that contribute to the control of cell type-specific gene regulation. In the pituitary cell nucleus, the homeodomain (HD) transcription factor Pit-1 orchestrates the activities of a network of regulatory proteins that control PRL gene expression. The broad objective of this proposal is to use emerging concepts in nuclear architecture and chromatin remodeling to determine how Pit-1 coordinates the activities of multi-protein complexes at specific gene enhancers and promoters. Pit-1 regulates PRL transcription through its interactions with other coregulatory proteins including the CCAAT/enhancer-binding protein alpha (C/EBP1). C/EBP1 in turn associates with the heterochromatin binding protein 1 alpha (HP11) in regions of centromeric heterochromatin, and Pit-1 can recruit C/EBP1 from the regions of compact chromatin. Disease-causing mutations in Pit-1 disrupt this network of protein interactions, and these results have broad implications for many human diseases linked to mutations in the HD proteins. The studies in this proposal use the combination of biochemical analysis and live-cell imaging to test the hypothesis that Pit-1 interactions with the C/EBP1-HP11 complex function to remodel densely packaged chromatin, allowing the access of pituitary-specific transcription factors to target genes. The first aim is to define the interactions of C/EBP1 and HP11 in regions of heterochromatin in pituitary cells, and then to determine the role of Pit-1 in regulating this network of protein interactions. The second aim is to determine how these interactions function to control local chromatin remodeling. The third aim will take advantage of newly developed transgenic mouse models that allow the unambiguous identification of living lactotrope or somatotropes cells to map the network of Pit-1 interactions in the normal mature mouse pituitary cells. Relevance: The anterior pituitary hormone PRL has many diverse physiological roles, and the failure to regulate PRL synthesis leads to reproductive disturbances in both men and women, and can lead to prolactinomas, the most common intracranial primary tumor. If we are to understand disease processes and design therapeutic strategies, it is important to define how specific gene regulatory complexes are assembled in the intact cell nucleus. Discovering how nuclear architecture controls gene expression will be the cornerstone for understanding how genomes work.